A known vane type valve timing control apparatus is disclosed in JP11-294121A. The valve timing control apparatus disclosed controls an opening and closing timing of valves of an internal combustion engine by a supply and a discharge of an operational fluid relative to a fluid chamber formed between a housing member and a vane rotor. The housing member is one of rotational members integrally rotating with a pulley or a sprocket, which synchronously rotates with a crankshaft of the internal combustion engine. The vane rotor is the other one of rotational members including a vane used for dividing the fluid chamber into two operational chambers and rotating on a radially inner side of the housing member. The vane rotor is provided so as to be coaxial and rotatable with the housing member, and integrally rotating with a camshaft of the internal combustion engine for opening and closing the valves of the internal combustion engine. The two operational chambers are equal to an advanced angle chamber displacing a relative rotational phase of the vane rotor to the housing in an advanced angle direction by a supply of an operational fluid to the advanced angle chamber, and a retarded angle chamber displacing a relative rotational phase of the vane rotor to the housing in a retarded angle direction by the supply of the operational fluid to the retarded angle chamber. The advanced angle chamber and the retarded angle chamber are separated from each other by means of the vane. Then, a fluid pressure in the advanced angle chamber and the retarded angle chamber is adjusted to thereby control the relative rotational phase between the housing member and the vane rotor. That is, in response to an operation state of the engine, a rotation of the camshaft relative to the crankshaft is controlled to thereby control an opening and closing timing of the valves. The controlling performance depends on a pressure receiving area and a volume of the fluid pressure chamber, and the like.
For example, the intake valve is controlled on a most retarded angle side at a start of the internal combustion engine, an idling driving state, and the like, and then controlled towards the advanced angle side in response to an increase of revolutions of the internal combustion engine. The operational fluid (for example, oil) is activated by a power of the internal combustion engine and is supplied by an oil pump having a suction capacity in response to the revolutions of the internal combustion engine. In the case of low revolutions of the internal combustion engine, the fluid pressure decreases and thus a sufficient pressure receiving area and the volume of the fluid pressure chamber are provided for ensuring necessary responsiveness.
On the other hand, when the internal combustion engine turns to a stable operation state, the intake valve should be appropriately controlled between the advanced angle side and the retarded angle side in response to the operation state of the engine. However, since the oil is used as lubricant of the internal combustion engine or a power transmission mechanism, the increase of temperature may cause decrease of viscosity of the oil. As a result, leakage may easily occur to thereby induce a decrease of a hydraulic pressure. Further, because of a pressure control valve normally provided at the hydraulic pressure system, all of the suction force of the oil pump increasing in response to the revolutions of the internal combustion engine may not be used. Accordingly, required operational responsiveness may not be obtained. In order to increase the responsiveness, it is effective to reduce the volume of the fluid pressure chamber. However, a torque generation may also be reduced to thereby deteriorate the control ability especially at low revolutions.
Thus, a need exists for a valve timing control apparatus which is not susceptible to the drawback mentioned above.